The invention concerns a method for manufacturing an electrical insulator and an apparatus for use with the method for manufacturing an electrical insulator.
Today, there are a number of different methods for manufacturing an electrical insulator for high voltages. The majority of these electrical insulators are today made in porcelain. One alternative to porcelain can be a hollow insulator that consists of a tube, preferably made of a composite material, an attachment flange at each end of the insulator and, as outer insulation, a coating, commonly of silicone rubber. Another electrical insulator can comprise a solid rod with an added outer insulation, commonly silicone rubber. In the text below, the terms tube/rod are designated body.
It is known that to manufacture an electrical insulator of the type described above, a silicone coating in the form of an inverted T-shaped-profile is wound in a screw-like fashion around a ready-made body. The coating is wound from one end of the body in a screw-like fashion so that one edge of the T-shaped profile comes to lie against the other edge of the T-shaped profile on the next turning. One problem with the known insulator is that of achieving a tight fit when attaching the inverted T-shaped profile against the body, especially with regard to the tightness between the joints of adjacent turns of the finished profile. One way of trying to solve these problems is described in FR-A1-2 726 684, which describes a method for manufacturing such an electrical insulator. In this case, the silicone rubber material is extruded from a nozzle at a distance from the body and is thereafter wound around the body. The profile is shaped so that the edges of the legs of the T-shaped profile are bevelled, so that when correctly wound, the edge of the profile of a subsequent turn incline inwards over the adjacent edge of the previous turn. A pressing tool then presses together the joints between each of the following winding turns to achieve sufficient tightness between the joints.
In spite of the intended improvements with the method according to the French patent application named above, the risk that insulating problems could arise at the joints of adjacent windings still remains, which finally could end up in the insulator to loosing its insulating properties. One problem that can arise is that the silicone does not extend all the way up against the flange, which can give rise to partial discharges at the flange and that can then cause an electrical flash-over. Another problem with previous techniques is that one could get trapped air between the wound profile and the body. Moisture can force its way into these air pockets, which can lead to the insulator loosing its insulating ability.
According to the known method for the manufacture of a spirally wound insulator, the silicone rubber profile is extruded in a linear line at a distance from the body, after which it is wound around the body.
When the profile is wound around the body, the outermost radial sections are exposed to stretching. This stretching can result in part in tension remaining in the material, and in part in deformities in the profile during winding.
The objective of the present invention is to achieve a new method that, by means of simple manufacturing process, achieves an insulator comprising a body, and with a very homogeneous and reliable exterior coating of the body, yet that is nevertheless very cost-efficient.
Another objective is to achieve a method that eliminates cavities and trapped air between the applied material and the body, as well as at possible flanges at the ends of the body.
A further objective of the invention is to achieve an apparatus for manufacturing an insulator in accordance with the method according to the invention.
As such, the method according to the invention includes the manufacture of an electrical insulator including a body and a coating of insulating material arranged on the body, whereby the coating is applied to the body in a screw-like or helical-like fashion where the coating is molded by extrusion directly on the body. Thus, a good adhesion bond is obtained with the outer surface of the body, and, additionally, a homogeneous joint with the adjacent turn is also obtained. In this way, a very homogenous coating with good adherence to the body is achieved. This method prevents the occurrence of trapped air pockets.
The invention even includes an apparatus for manufacturing an insulator that includes a nozzle for extruding an insulating material to be applied onto a body, and an apparatus for displacing the mutual positions of the nozzle and the body, and where the body is arranged to form an additional complementary part of the extrusion nozzle.